The present invention relates to machine tools, and more particularly, the construction of machine tool frames.
The construction of machine tools and precision mechanical components (hereinafter referred to as machine tools) requires a material which can withstand high stress, be shaped with good accuracy, take a good finish when machined, carry load without significant deformation, and remain dimensionally stable over long periods of time. Materials which have these properties and are most commonly used for these applications are steel and cast iron.
Although these materials have the required properties for the construction of machine tools, they are all relatively expensive. Also, the process of casting iron, steel, or other metals into large precision mechanical forms is well known to be an expensive and difficult task. Because of the high temperatures and rapid differential cooling inherent in metal casting, significant internal stresses result. Much attention must be given to the balancing and relieving of these stresses to obtain dimensional accuracy during surface finishing operations and maintaining dimensional stability over the part's useful life.
In addition to care in manufacture, reliable operation from metal machine tools requires that they be securely mounted on and bolted to a rigid foundation which in almost every case is a thick concrete slab with anchor bolts cast into it to receive the leveling screws on the machine tool. This combination of leveling screws and jacking bolts is then used to "pull" the metal machine into level alignment. All but the smallest machine tools will be twisted and bent under their own weight and residual casting stresses relieved non-uniformly by machining of the ways until they are properly installed on the rigid foundation. It is safe to say that overall rigidity and resistance to forces generated by the tool's operation is primarily carried by the foundation. It is only local stresses which the metal frame handles and transmits to the foundation. The surface finish necessary to support lubrication between moving parts is also conveniently produced directly on a metal frame.
All prior applications of concrete to machine tool construction amount to attempts at changing the location of the mechanical interface between the local stress carrying metal members and the overall load carrying concrete foundation. In some instances concrete is used to fill otherwise hollow metal castings to increase stiffness and damp vibration. In the extreme, metal ways are attached to the concrete creating a "concrete frame".
The only material other than iron and steel in somewhat common usage for construction of machine tools is granite. However, the predominate use of granite in the machine tool industry is not for tool frames but rather as surface plates. Surface plates are nothing more than a very accurately flat surface used as an inspection stand or flat reference for precise measurement. The use of granite in machine tool construction is a conceptual extension of granite surface plates and is really an extreme measure, involving great difficulty and very high costs and is resorted to only as a last measure where only the well-known properties of granite can result in a rigid enough set-up to produce really the ultimate in cutting accuracy. The difficulties in using granite and the distinction from concrete are more apparent than between concrete and cast iron.
Of course, the biggest difference is that granite is not cast to shape, rather it is quarried. A single, flawless block of granite the size of the machine desired must be cut, then the desired shape cut from such a block. Even after the general shape is obtained, extreme measures are required to attach anything to the granite in order to create a usable tool.
The state of the art as discussed above is illustrated by the following patents.
Briese, U.S. Pat. No. 3,618,432, discloses construction of a vibration-free lathe. The lathe is preferably made from a single monolithic block of granite, although it is stated that other naturally occurring minerals or rock may be equally satisfactory as may also be selected synthetic materials.
Zagar, U.S. Pat. No. 3,800,636, discloses a compression reinforced concrete machine tool frame. The concrete is molded with the tensioning rods in place, which rods are tensioned after the concrete has set. The machine tool components can be mounted on or anchored within the concrete frame. Other machine tool components which are moveable slide on steel runners which are likewise mounted on the frame. Zagar also discloses that a release component (e.g. grease, soft wax, paraffin) may be coated onto the tensioning rods to prevent them from being anchored to the frame.
Yeomans et al, U.S. Pat. No. 2,487,289, discloses a machine tool which appears from FIG. 1 to have a cementitious base, although there is no discussion of specific materials.
Nenninger et al, U.S. Pat. No. 2,010,557, also discloses a machine tool having a concrete base for minimizing vibration and to prevent deflection of a machine tool bed.
Accordingly, a need existed in the art for a process for constructing machine tools from inexpensive and easily used materials, while still obtaining adequate surface finish to sustain lubrication.